MIDDLE–LATE ORDOVICIAN (DARRIWILIAN–SANDBIAN) PAIRED SULFUR AND CARBON ISOTOPE STRATIGRAPHY: LINKS TO PULSES OF OCEAN OXYGENATION, WEATHERING, AND BIODIVERSITY

Major phases of the Great Ordovician Biodiversification Event occurred during the Darriwilian (468 to 460.5 Ma) Stage, and this unprecedented change in biodiversity and biocomplexity of marine life has been linked to an asteroid breakup event and to sea surface temperatures that cooled to modern equatorial ranges. Ocean oxygenation events have also been linked to large diversification events of marine life, from the Neoproterozoic to the late Cambrian. Although recent sulfur and carbon isotope studies of Early to Middle Ordovician sequences in Newfoundland and the Argentine Precordillera have highlighted evidence for persistent widespread euxinic (anoxic, sulfidic) deep marine waters, there is very limited amount of sulfur isotope data from the Darriwilian Stage of the Ordovician. A positive δ¹³C excursion has been globally recognized in the middle Darriwilian Stage and coincides approximately with a major Ordovician lowstand (Sauk-Tippecanoe sequence boundary).

Here we present carbonate-associated sulfate (δ³⁴S_CAS) and pyrite (δ³⁴S_pyrite) paired with carbonate δ¹³C_carb and organic matter δ¹³C_org from expanded Middle to Upper Ordovician sequences from the Appalachian Basin and Arbuckle Mountains regions of North America. Two major negative shifts in δ³⁴S_CAS of 13‰ are documented, the oldest occurring within the holodentata–polonicus Conodont Zones and the younger drop occurring within the sweeti–tvaerensis (gerdae subzone) Conodont Zones. These negative shifts in δ³⁴S_CAS have an antithetical relationship to positive shifts in δ³⁴S_pyrite and δ¹³C_carb values. Geochemical box modeling of these sulfur isotope shifts suggest that a decrease in the global rate of pyrite burial for ~1 Ma or increasing the weathering flux of pyrite to the oceans would lead to these sulfur isotope trends. A decrease in pyrite burial rates, likely due to ocean oxygenation, would have further ventilated marine environments and facilitated the major phases of biodiversification. Likewise, increased weathering is consistent with a sea-level lowstand and the seawater ⁸⁷Sr/⁸⁶Sr isotope record in the late Darriwilian. Regardless of ultimate cause(s), further modeling and isotope stratigraphy studies are needed to elucidate the connectivity of ocean/atmospheric chemical changes to marine biodiversity in the Ordovician.